Carboxylic acids are important chemicals of commerce. They appear as desired or contaminating constituents of a wide range of aqueous process streams. Historically, they were produced from animal fat or vegetable oil sources or from petroleum sources in substantially nonaqueous systems. More recently, they have been identified among the most attractive products for manufacture from biomass (e.g., corn starch) by fermentation. In these more advanced processes, the carboxylic acid is generated as a dilute solution in an aqueous fermentation broth. Acetic acid is recovered commercially from dilute aqueous solutions by distillation or by extraction with solvents such as isopropyl acetate, other esters, or ethers. Aqueous solutions are created during the manufacture of adipic acid. Citric acid is recovered from fermentation broths commercially by solvent extraction with high-molecular-weight tertiary amines (e.g., tridecylamine) in a diluent composed of a hydrocarbon (e.g., kerosene) and an alcohol (e.g., n-decanol). Citric acid commands a substantial market, which is increasing as detergent manufacturers switch to citric acid as "builder". Lactic acid (raw material for biodegradable plastics), succinic acid, malic acid, fumaric acid, and other carboxylic acids which may be manufactured on a large scale by fermentation of biomass are creating considerable interest in solvent extraction as a means of recovery. Carboxylic acids are also stable oxidation products and frequently appear as by-products or contaminants in aqueous and organic waste streams.
There are numerous current and potential industrial and environmental applications where it is desirable to recover these and other carboxylic acids from aqueous solutions. Examples include the production of citric acid and other acids by fermentation (Lockwood, 1979; Busche, 1985) and removal and recovery of carboxylic acids from aqueous waste streams. (All references noted herein are listed below in a section of the specification entitled "References.") For volatile carboxylic acids, such as acetic, distillation and azeotropic or extractive distillation are alternatives, along with solvent extraction and adsorption (King, 1983; Kuo et al., 1987). For low-volatility carboxylic acids, e.g., dicarboxylic acids and hydroxycarboxylic acids, distillative processes are expensive and often cannot isolate the desired acid.
For acids such as citric and lactic, the classical approach for recovery from a fermentation broth has been to add calcium hydroxide to form the calcium salt of the carboxylic acid, to which an acid such as sulfuric is added to liberate the free carboxylic acid. This approach consumes chemicals (e.g., lime and sulfuric acid) and produces a waste salt stream. Consequently, this method is falling out of favor.
B. Urbas, in U.S. Pat. No. 4,405,717 and No. 4,444,881 teaches a process for recovering acetic acid, lactic acid, butyric acid and citric acid directly from fermentation broths. This process involves converting the acid to a calcium salt and then adding a tertiary amine carbonate (especially tributylamine carbonate) to give a trialkylammonium salt of the acid and calcium carbonate. The trialkylammonium carboxylate is heated to give the acid and the corresponding trialkylamine. This process has the disadvantage that it generates calcium carbonate, a solid waste that needs to be disposed of or heated to high temperatures in a kiln to convert it back to calcium oxide. Also in these patents, there is a preference for higher molecular weight amines and the use of distillation to remove volatile acids from the less volatile amines.
Solvent extraction is often effective for recovery of these low-volatility carboxylic acids from aqueous solution. Reactive, basic extractants, e.g., tertiary amines or phosphine oxides, can be used to gain greater solvent capacity and selectivity with respect to water and other species. A process developed by Miles, Inc. (Baniel et al., 1981) for recovery of citric acid from fermentation solutions uses a solvent composed of a tertiary amine extractant in a hydrocarbon diluent with an alcohol modifier. This extractant is regenerated by back-extraction of the acid into water at a higher temperature. Back-extraction following a shift in diluent composition, achieved, e.g., by distillation, is another possibility for regeneration, and can be combined with a swing of temperature (Tamada and King, 1990). The overall degree of concentration relative to the feed that can be achieved by these methods is limited by the extent to which the distribution equilibrium for the carboxylic acid can be changed between forward and back-extraction.
Ion exchange and adsorption have also been employed in carboxylic acid recovery schemes. U.S. Pat. No. 4,720,579 to Kulprathipanja discloses the use of styrene-divinylbenzene resins to adsorb citric acid with regeneration by water or by a mixture of acetone and water. Similarly, Great Britain Patent No. 2,064,526A discloses the use of adsorbents containing pyridyl functional groups combined with regeneration by leaching with an organic solvent such as an alcohol or a ketone. U.S. Pat. No. 4,924,027 to Kulprathipanja and Strong discloses adsorption of citric acid by adsorbents containing tertiary amine or pyridyl functionalities (including Bio-Rad AG3-X4A and AG4-X4), with regeneration using an aqueous solution of sodium, potassium or ammonium hydroxide, yielding the respective sodium, potassium or ammonium citrate. Treatment of these citrates with a strong acid would yield the free citric acid form. In each of these solutions the citric acid is adsorbed from an aqueous solution bekow the pKa of citric acid.
As can be seen from this description of background, various methods used heretofore to recover carboxylic acids have presented limitations and thus offer opportunities for improvement. It is accordingly a general object of the invention to provide an efficient process for the recovery of carboxylic acids from aqueous solutions which neither consumes large amounts of chemicals nor generates waste chemical streams.